Hitherto, a member in which a terminal (a connector) made of copper or a copper alloy (for example, brass) is attached to electrical wires comprised of conductors of copper or a copper alloy, which is called a wire harness, has been used as an electrical wiring for movable bodies, such as automobiles, trains, and aircrafts. Among a means of weight reduction required for movable bodies in recent years, studies have been progressing on use of aluminum or an aluminum alloy that is lighter than copper or a copper alloy, as a conductor for the electrical wiring, in place of the copper or the copper alloy.
The specific gravity of aluminum is about one-third of that of copper, and the electrical conductivity of aluminum is about two-thirds of that of copper (when pure copper is considered as a criterion of 100% IACS, pure aluminum has about 66% IACS). Therefore, in order to pass an electrical current through a conductor wire of pure aluminum, in which the intensity of the current is identical to that through a conductor wire of pure copper, it is necessary to adjust the cross-sectional area of the conductor wire of pure aluminum to about 1.5 times larger than that of the conductor wire of pure copper. Nevertheless, an aluminum conductor wire is still more advantageous in mass than a copper conductor wire, since the former has an about half weight of the latter.
Herein, the term “% IACS” mentioned above represents an electrical conductivity when the resistivity 1.7241×10−8 Ωm of International Annealed Copper Standard is defined as 100% IACS.
There are some tasks in using aluminum as a conductor of electrical wirings for movable bodies.
One of the tasks is improvement in resistance to bending fatigue. This is because a repeated bending stress is applied to a wire harness attached to a door or the like, due to opening and closing of the door. A metal material, such as aluminum, is broken at the certain number of times of repeating of applying a load when the load is applied to or removed repeatedly as in opening and closing of a door, even at a low load at which the material is not broken by one time of applying the load thereto (fatigue breakage). When the aluminum conductor is used in an opening and closing part, if the conductor is poor in resistance to bending fatigue, it is concerned that the conductor is broken in the use thereof, to result in a problem of lack of durability and reliability. Generally, a material higher in mechanical strength is said to have more satisfactory resistance to bending fatigue. Thus, it may be considered that it might be favorable to apply an aluminum wire high in mechanical strength. However, on the other hand, a worked product high in mechanical strength is insufficient in elongation, and it is difficult to conduct the installation to vehicles. Thus, in general, annealed products that can secure elongation are utilized in many cases.
Another task is enhancement of tensile strength. This is to maintain the tensile strength of a crimp section in a connection part of a wire and a terminal, and to endure a load that is abruptly applied thereto in installation to a vehicle. Since replacement of copper conductors with aluminum conductors results in an increase in the cross-sectional area as described above, the resistance to applied load [N] is apt to increase. However, a pure aluminum conductor is lower in resistance to applied load [N] than a copper conductor, and it is difficult to replace the copper conductor. Thus, there is a demand for a new wire of an aluminum conductor, which is improved in resistance to applied load per unit area (tensile strength [MPa]).
According to the above, for an aluminum conductor that is used in electrical wirings of movable bodies, a material is required, which is excellent in tensile strength and resistance to bending fatigue, as well as which is excellent in electrical conductivity that is required for passing much electricity.
For applications for which such a demand is exist, ones of pure aluminum-based alloys represented by aluminum alloy wires for electrical power lines (JIS 1060 and JIS 1070) cannot sufficiently tolerate a repeated bending stress that is generated by opening and closing of a door or the like. Further, although an aluminum alloy in which various additive elements are added is excellent in tensile strength, the alloy has such problems that the electrical conductivity is lowered due to solid-solution phenomenon of the additive elements in aluminum, and that wire breaking occurs in wire-drawing due to formation of excess intermetallic compounds in aluminum. Therefore, it is necessary to limit and select additive elements, to prevent wire breakage as an essential feature, as well as to prevent lowering in electrical conductivity, and to enhance mechanical strength and resistance to bending fatigue.
Typical aluminum conductors used in electrical wirings of movable bodies include one described in Patent Literature 1. This is to realize the required tensile strength, elongation at breakage, impact resistance, and the like, by using an electric wire conductor that is formed by stranding a plurality of fine aluminum alloy solid wires.
However, since the aluminum conductor described in Patent Literature 1 is large in a grain size, the aluminum conductor does not satisfy resistance to bending fatigue, and a further improvement has been demanded.